Ordinary metals in their solid state assume a crystalline texture. Under special conditions (alloy composition and sudden cooling and solidification), even in their solid state, they acquire an atomic structure which, similarly to a liquid, does not contain any crystalline texture. Metals and alloys which possess such an atomic structure are called amorphous. When such an amorphous alloy is made of component elements selected suitably and used in proper proportions, it will excel conventional practical crystalline metal materials in chemical, electromagnetic, physical, mechanical properties, and the like. Accordingly, such a material has a high possibility of finding extensive utility in applications such as electrical and electromagnetic parts, composites, and textile materials. Amorphous alloys possessing high magnetic permeability are disclosed in Japanese patent application (OPI) Nos. 73920/76 and 35618/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), amorphous alloys excelling in strength, corrosionproofness, and thermal resistance are disclosed in Japanese patent application (OPI) Nos. 101215/75 and 3312/76; and typical amorphous alloys excelling in thermal stability are disclosed in Japanese Patent Publication No. 19976/80 (U.S. Pat. No. 3,856,513). Among the amorphous alloys which have various outstanding characteristics as described above, iron-based alloys are characterized by low prices of raw materials available, high degrees of tensile strength at fracture as compared with conventional practical crystalline metal materials, virtual absence of work hardening, and outstanding toughness. Therefore, they prove useful as materials for a wide variety of industrial products such as reinforcing agents, complexing agents, fibrous materials, etc. Among other amorphous iron-based alloys, Fe-S-B type alloys possess high tensile strength at fracture reaching a maximum even exceeding 400 kg/mm.sup.2. Further, the Fe-Si-B type alloys have been known as amorphous iron-based alloys possessing unusually high degrees of thermal resistance as compared with other iron-metalloid type alloys. From the standpoint of the practical utility of metal materials, in the case of the materials used in the parts on which external forces act statically, their properties are evaluated with emphasis on the results of tensile test, particularly those on the tensile strength at fracture. In the case of the materials for belts, tires, ropes, and machine parts which produce rotating or reciprocating motions at high rates of speed (dynamic materials), however, the results of test for tensile strength, particularly those on the tensile strength at fracture, do not deserve any attentive consideration. This is because forces repetitively act on these materials for long periods of time and, in many cases, inevitably entail such phenomena as vibrations. Accordingly, actual fractures occur in these materials without such heavy deformation as would be observed in the test for tensile strength. These fractures induce fatigue breaking under much lower stress than the tensile strength at fracture or even the yield point. This fatigue property is the most important attribute for dynamic materials. If a given dynamic material possesses outstanding tensile strength at fracture, it still cannot be advantageously utilized unless it is also excellent in the fatigue property. As regards mechanical properties of amorphous alloys, the results of the tensile test and the compression test performed on a wide variety of alloys have been reported in a number of publications. Concerning the study on the fatigue property which is important from the practical point of view, the results obtained by Masumoto, Ogura, et al., on Pd.sub.80 Si.sub.20 amorphous alloy ribbons (Scripta Metallugica, Vol. 9, pp. 109-114, 1975) and those obtained by Imura, Doi, et al., on Ni-based, Febased, and Co-based amorphous alloy ribbons (Jpn. J. Appl. Phys., 19, 449, 1980 and Jpn. J. Appl. Phys., 20, 1593, 1981) are about all the reports found in literature. From the results of the study by Imura, Doi, et al., it is noted that the Fe.sub.75 Si.sub.10 B.sub.15 amorphous alloy ribbons possessing high strength showed the same level of fatigue property as the existing crystalline SUS 304 and registered a fatigue limit, .lambda.e=0.0018. This means that the amorphous alloy ribbons of Fe.sub.75 Si.sub.10 B.sub.15 shows no appreciable improvement in fatigue property for its high tensile strength at fracture and exhibits rather low fatigue ratio as compared with counterpart materials now in practical use.
Japanese patent application (OPI) No. 4017/76 discloses an amorphous iron alloy which has as its main component an Fe-(P, C, B)-Cr type alloy intended primarily for improvement of corrosionproofness (resistance to surface corrosion, resistance to pitting, resistance to interstitial corrosion, and resistance to stress-corrosion cracking) and additionally as a secondary component varying elements. This alloy is claimed to be useful for preparation of reinforcing cords to be buried in rubber and plastic products such as automotive tires and conveyor belts. This patent application claims a patent for an amorphous iron alloy possessing high strength and stability to resist fatigue, surface corrosion, pitting, interstitial corrosion, stress-corrosion cracking, and hydrogenation embrittlement, which amorphous iron alloy contains as main components thereof 1 to 40 atom% of Cr and 7 to 35 atom% of at least one element selected from among P, C, and B, further contains as a secondary component thereof at least one of the following four members:
(1) 0.01 to 40 atom% of either or both of Ni and Co, PA0 (2) 0.01 to 20 atom% of at least one element selected from the group consisting of Mo, Zr, Ti, Si, Al, Pt, Mn, and Pd, PA0 (3) 0.01 to 10 atom% of at least one element selected from the group consisting of V, Nb, Ta, W, Ge, and Be, and PA0 (4) 0.01 to 5 atom% of at least one element selected from the group consisting of Au, Cu, Zn, Cd, Sn, As, Sb, Bi, and S
in a combined amount falling in the range of 0.01 to 75 atom%, and has the balance to make up 100 atom% substantially of Fe. The alloy which is specifically disclosed in Japanese patent application (OPI) No. 4017/76 is in a composition of Fe.sub.67 Cr.sub.3 Si.sub.15 B.sub.1 P.sub.13 C.sub.1, thus using Fe-Si-P-Cr as its main components. Although this alloy excels in corrosionproofness (resistance to surface corrosion, resistance to pitting, resistance to interstitial corrosion, and resistance to stress-corrosion cracking), it possesses very poor amorphous texture forming ability and exhibits no appreciably improved fatigue property. Thus, the alloy falls short of being useful as the dynamic materials defined above.
The inventors of this invention formerly filed a patent application covering a filament of circular cross section made of an amorphous iron-based alloy excelling in corrosionproofness, toughness, and electromagnetic property and useful as industrial materials for the production of electric and electronic parts, composites, and textile articles and to a method for the manufacture of the filament (U.S. Ser. No. 254,714 and EPC Disclosure No. 39169). In some of the working examples cited in the specification thereof, Fe.sub.71 Cr.sub.10 Si.sub.10 B.sub.9 alloy, Fe.sub.70 Cr.sub.5 Si.sub.10 B.sub.15 alloy and Fe.sub.50 Co.sub.20 Cr.sub.5 Si.sub.10 B.sub.15 alloy resulting from addition of Cr to the Fe-Si-B type alloy composition are indicated. The addition of Cr in the prior art is aimed at improving thermal resistance and strength, but it is not aimed at fatigue property. In the possible alloy compositions contemplated by this patent application, the Fe.sub.70 Cr.sub.5 Si.sub.10 B.sub.15 alloy and Fe.sub.50 Co.sub.20 Cr.sub.5 Si.sub.10 B.sub.15 alloy which incorporate 5 atom% of Cr show practically no discernible improvement in fatigue property and the Fe.sub.71 Cr.sub.10 Si.sub.10 B.sub.9 alloy which incorporates 10 atom% of Cr possesses poor amorphous texture forming ability.